Common Syntax Errors In Java And The Ultimate Guide To Avoid Them

You must be meticulous when writing code to avoid errors and make sure your programs work, especially when it comes to the syntax of your chosen coding language. Syntax errors in Java may be particularly difficult if you’re not used to the language. Java is not known to be accessible or beginner-friendly compared to a language such as Python. Making mistakes, especially syntax mistakes, is common.

There are several common syntax errors in Java you may run into while programming. When you know what to expect, you can take measures to avoid these syntax errors in Java and other mistakes.

What Are Syntax Errors in Programming?

Every programming language has unique rules of grammar called its syntax, though some may share similarities with other languages. The syntax of a programming language dictates what a valid expression or statement looks like. The code will only run if you write it with a valid syntax. Otherwise, the code’s compiler or interpreter will not understand the statement, generating a syntax error.

High-level languages (Java, C++, and Python) have more human-readable syntax compared to low-level languages, such as assembly language, which have less abstraction.

Take the following code snippets written in several programming languages. In each example, a simple “find” program designed to locate a word in a sentence uses “if” statements and built-in functions such as “in,” “find,” and “contains.” If the target word is found, a sentence is printed in the console, indicating that the computer program found the word.

Java syntax

String mySentence = “The quick brown fox jumped over the lazy dog.”;
String wordToFind = “fox”;
if (mySentence.contains(wordToFind)) {
System.out.println(“I found the word ‘” + wordToFind + “‘ in the sentence.”);
}

Python syntax

my_sentence = “The quick brown fox jumped over the lazy dog.”
word_to_find = “fox”
if word_to_find in my_sentence:
print(f”I found the word ‘{word_to_find}’ in the sentence.”)

C++ syntax

#include <iostream>
#include <string>
using namespace std;
int main() {
string mySentence = “The quick brown fox jumped over the lazy dog.”;
string wordToFind = “fox”;
if (mySentence.find(wordToFind) != string::npos) {
cout << “I found ‘” << wordToFind << “‘ in the sentence.” << endl;
}
return 0;
}

Each language writes the program with unique syntax even though they perform the same task (in this case, looking for a word in a sentence). Note how sentences are structured, the indentations, and the symbols used. These elements make up the syntax for a programming language.

You can see some of the differences in the syntax of each language in these short examples. The differences are even more apparent when looking at longer, much more complex programs.

The differences between languages in the examples above include:

• Declaration of variables: Dynamic typing is allowed in Python, with the string “my_sentence” not explicitly declared. In Java and C++, the string type must be declared: “String mySentence” and “string mySentence.”
• Naming conventions: Python has all lowercase names with underscores instead of spaces — known as snake case. Java and C++ lowercase the first word, have no spaces, and capitalize words after the first word — known as camel case.
• Indentation: Python uses indentation for each code block, whereas Java and C++ use curly braces.
• Punctuation: Java and C++ require semicolons to end each line, while Python does not.
• Printing to the console or terminal: The syntax for printing to the console differs, as shown in the last few lines of each example above.

If you write a program in one language and forget to use the symbols and syntax required, you will get a syntax error if you try to run the program.

For example, say you forget the colon at the end of the “if” statement in a Python program. If you are running the program in Integrated Development and Learning and Environment (IDLE), you will get a syntax error that indicates what was expected and on which line the error was found, as shown in the following image.

What Are Syntax Errors in Java?

When you write and run a program in Java, your computer will first attempt to compile and interpret the code you’ve written so that the code is in a format the computer can understand. The Java Compiler will alert you of syntax errors in Java programs before they are completely compiled and run.

The Java Compiler first compiles Java code into bytecode. The Java Virtual Machine (JVM) then converts the bytecode into a usable program that can be run on the local hardware.

This can be done on different systems. Java is a multiplatform language that can run on desktop platforms, mobile platforms, web browsers, and any platform with a JVM installed. You don’t have to write a different program from Windows, Mac, web browsers, or anywhere else you want to run your code.

For example, if you want to write a web-scraping app in Java, you can use that app on multiple platforms without changing the code.

If you do not write your code using the appropriate Java syntax, you will get syntax errors in the Java programs you attempt to run, regardless of what platform you use.

Examples of syntax errors in Java code

Some common Java syntax errors you may run into while programming Java include:

• Undeclared variables
• Missing colons, brackets, or parentheses
• Omitting a semicolon at the end of a line
• Misspelled keywords
• Incorrect capitalization (Java is case-sensitive)
• Mismatched quotes, such as starting a string with a single quote and ending in a double quote
• Variable name errors, such as starting a variable with a number
• Incorrectly declaring methods, such as not using curly braces
• Incorrect use of mathematical operators
• Invalid type casting (attempting to convert one type to another)
• Incorrect data types, such as int, float, or boolean
• Improper control structure for statements such as if, else, or switch statements
• Failing to import a class
• Omitting the word “break” in a switch statement
• Forgetting to include a “return” statement

These are just a few of the syntax errors in Java programming. If you’re learning the language, make sure to study it closely to avoid making similar syntax errors in Java.

Java Errors and Solutions

Before you can solve Java errors, you need to make sure you have your system properly set up in the first place. Java programming requires one of the latest Java Editions, such as the Java Standard Edition (Java SE), the Java Enterprise Edition (EE), or the Java Micro Edition (ME). You will also need the Java Development Kit (JDK) and an interactive development environment (IDE) like Eclipse or JetBrains to write your code. If you have all of the above correctly installed on your system, you should be able to write and run Java programs.

Debugging your code in the IDE is a useful method for catching syntax errors in Java programs. You can run and debug the following code in Eclipse.

After making a new project and class, you will have your main function into which you can write a common such as “print line” to output a line of text into the console window of Eclipse:

package com.myprogram;
public class myClass {
public static void main(String[] args) {
// TODO Auto-generated method stub
System.out.println(“Hello, world”);
}
}
You can run a simple “Hello, world” program to make sure everything works. Your IDE will alert you if anything is wrong with your package or the code in your Main class and recommend a solution.

Using an IDE such as Eclipse to write your program will alert you of issues that need to be resolved, as it will not run code that contains syntax errors in Java. Eclipse notifies you of syntax errors in Java as you type them.

The string “Hello, world” is the expected output to either the terminal or console when you click Run or Debug from the Run menu or choose Run [package name] from the menu toolbar. If you forget the semicolon at the end of the line with the print line function “System. out.println(“Hello, world”),” a red line appears under the mistake, and there is an error indicator next to the line number. If you hover over either the red line or the error indicator, you will see the error message: “Syntax error, insert “;” to complete BlockStatements.”

The IDE will warn you of syntax errors in Java as you write before you even debug the program. Simply hover over the error indicator or the red line under the error for a solution.

Syntax errors in Java web scraper code

You may run into several syntax errors in Java when web scraping. Consider the following Java code for a basic web scraper program that finds the title of a web page and exports it into a text document. It is a longer and more complex program than the earlier examples. It’s not uncommon to have more than one syntax error in Java programs like this.

package com.example.scraper;
import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class Scraper {
public static void main(String[] args) {
String url = “https://lite.cnn.com”;
try {
URL cnnUrl = new URL(url);
HttpURLConnection connection = (HttpURLConnection) cnnUrl.openConnection();
InputStream inputStream = connection.getInputStream();
InputStreamReader myReader = new InputStreamReader(inputStream);
Scanner myScanner = new Scanner(myReader);
BufferedWriter myWriter = new BufferedWriter(new FileWriter(“C:/path/to/file/title.txt”));
Pattern myTitlePattern = Pattern.compile(“<title>(.*?)</title>”);
while (myScanner .hasNextLine()) {
String line = myScanner .nextLine();
Matcher matcher = myTitlePattern.matcher(line);
while (matcher.find()) {
String titleText = matcher.group(1);
myWriter .write(titleText);
myWriter .newLine();
}
}
myWriter .close();
myScanner .close();
System.out.println(“Scraping complete.”);
} catch (IOException e) {
e.printStackTrace();
}
}
}
This code scrapes the website https://lite.cnn.com, a condensed and simplified version of the CNN news homepage. The program will find the title of the page and save it to a text file.

The following are some examples of syntax errors in Java code you may run into in the example program above.

Missing curly brace

If you forgot a curly brace opening up the main method and you ran the program in Eclipse, you will get this syntax error:

Exception in thread “main” java.lang.Error: Unresolved compilation problem:

Syntax error on token”)”, { expected after this token

at Scraper/com.example.scraper.Scraper.main(Scraper.java:15)

Missing parentheses

If you forget a parenthesis, you will get the following syntax error:

Exception in thread “main” java.lang.Error: Unresolved compilation problem:

Syntax error on token “Scannerreader”, ( expected after this token

at Scraper/com.example.scraper.Scraper.main(Scraper.java:24)

Missing equals sign

If you leave out an equals sign, you will get this syntax error:

Exception in thread “main” java.lang.Error: Unresolved compilation problem:

Syntax error on token “connection”, = expected after this token

at Scraper/com.example.scraper.Scraper.main(Scraper.java:20)

Undeclared variables

If you don’t declare the variable “URL” to be a String, you’ll get the following error:

Exception in thread “main” java.lang.Error: Unresolved compilation problems:

url cannot be resolved to a variable

at Scraper/com.example.scraper.Scraper.main(Scraper.java:16)

Misspelled keywords

If you misspell a keyword, such as the word “Pattern,” on line 29, you will get the following error:

Exception in thread “main” java.lang.Error: Unresolved compilation problem:

Patern cannot be resolved

at Scraper/com.example.scraper.Scraper.main(Scraper.java:29)

Assignment errors

If you leave out the left part of an assignment, such as the word “if” on line 31, you will get syntax errors such as this error:

Exception in thread “main” java.lang.Error: Unresolved compilation problems:

The left-hand side of an assignment must be a variable

Syntax error, insert “AssignmentOperator Expression” to complete Assignment

Syntax error, insert “;” to complete Statement

at Scraper/com.example.scraper.Scraper.main(Scraper.java:31)

Double quotes for strings missing

Strings need to be surrounded by double quotes, such as the string of the website you want to scrape on line 16. If you leave them out, you will get an “unresolved” syntax error in Java:

Exception in thread “main” java.lang.Error: Unresolved compilation problems:

https cannot be resolved to a variable

Syntax error on token “:”, ; expected

at Scraper/com.example.scraper.Scraper.main(Scraper.java:16)

“Try and catch” syntax error

In the try-and-catch block of the program, you need to have the “catch” part of the statement. Whether you misspell the word or leave it out altogether, you will get a syntax error:

Exception in thread “main” java.lang.Error: Unresolved compilation problem:

Syntax error on token “caught”, catch expected

at Scraper/com.example.scraper.Scraper.main(Scraper.java:47)

Parameter error

On the same line, the parameter “e” is expected in the “catch” statement. A syntax error will occur if you leave it out or misspell the parameter:

Exception in thread “main” java.lang.Error: Unresolved compilation problems:

Syntax error, insert “VariableDeclaratorId” to complete FormalParameter

e cannot be resolved

at Scraper/com.example.scraper.Scraper.main(Scraper.java:47)

Case sensitive error

Java is case-sensitive, so you will get an error if you lowercase “System” on line 45:

Exception in thread “main” java.lang.Error: Unresolved compilation problem:

system cannot be resolved

at Scraper/com.example.scraper.Scraper.main(Scraper.java:45)

Type mismatch error

In Java, you need to declare a variable type when initializing variables. If you put “int,” short for integer, instead of “String” for the string “URL,” you will get a type mismatch error:

Exception in thread “main” java.lang.Error: Unresolved compilation problems:

Type mismatch: cannot convert from String to int

The constructor URL(int) is undefined

at Scraper/com.example.scraper.Scraper.main(Scraper.java:16)

Misspelled variable

If you misspell or otherwise use the wrong variable name for a variable that you have declared, you will get an unresolved compilation problem. For example, using the variable “writers” on line 42 without declaring a variable called “writers:”

Exception in thread “main” java.lang.Error: Unresolved compilation problem:

writers cannot be resolved

at Scraper/com.example.scraper.Scraper.main(Scraper.java:42)

Invalid assignment operator

If you attempt to assign a variable, such as “c = a /b,” but you do not include the proper assignment operator “=”, you will get an invalid assignment operator syntax error:

Exception in thread “main” java.lang.Error: Unresolved compilation problem:

Syntax error on token “/”, invalid AssignmentOperator

at Scraper/com.example.scraper.Scraper.main(Scraper.java:24)

Invalid mathematical function errors

Syntax errors in Java are often caused by misspelling a word or typing the wrong symbol, including a mathematical operator. If you make an error in a math equation (such as accidentally dividing an integer by zero because you put the wrong variable in the wrong place), you will get an arithmetic error:

Exception in thread “main” java.lang.ArithmeticException: / by zero

at Scraper/com.example.scraper.Scraper.main(Scraper.java:24)

Splitting a string on two lines

If you attempt to split a string on more than one line without properly concatenating it (by breaking the string into two with a plus sign between them), you will get a syntax error. For example:

Exception in thread “main” java.lang.Error: Unresolved compilation problem:

Syntax error on token “”to/headlines.txt””, delete this token

at Scraper/com.example.scraper.Scraper.main(Scraper.java:27)

Mistyping the main method

If you misspell or make an error writing the exact words Java expects when writing the header for the main method, you will get a syntax error explaining that the main method should look exactly like this: “public static void main(String[] args)” with no variation. For example:

Error: Main method is not static in class com.example.scraper.Scraper, please define the main method as:

public static void main(String[] args)

You can see in the above examples that when there are syntax errors in Java programs, your IDE:

• Gives you the error name, such as “Unresolved compilation problem: Syntax error.”
• Provides what is the best solution, such as “{ expected after this token.”
• Tells you the location of the error — for example, “Scraper.java:20,” meaning the error is on line 20 of the program.

You can fix the syntax errors in Java by noting the location of the errors and making the recommended changes.

The above errors are just a few syntax errors in Java you may run into. There are many resources out there to help you learn more about syntax errors in Java and other common issues.

Other types of errors

Syntax errors in Java are only one type of error. Some other errors in Java you may run across include:

• Java: error: release version 19 not supported error: This error means you are targeting a Java version, such as version 19, not supported by your JDK.
• .Class expected error in Java error: You may get this syntax error if you make a mistake with a class or object.
• Java: error: release version 5 not supported: Make sure you have the latest Java version supported by your JDK.
• Run-time error Java error: Run-time errors are exceptions that occur while the program is running, as opposed to syntax errors in Java programs that occur during compilation.

Syntax errors in Java are relatively easy to solve because they are caught before the program is fully compiled. This allows you to fix the error on the fly, compared to errors like the ones above, which often make it past compilation.

One way to catch all errors in Java is to put your entire program inside a try-and-catch block. Try-and-catch blocks do not catch syntax errors because the compiler or interpreter would catch the syntax errors in Java before the program could be run. But they can be useful for other errors.

The “try and catch” block in the example web scraper code from earlier starts at line 19 and includes all of the code up to line 46. Immediately after the word “catch” in line 46 is the line of code that will run if there is an error, known as an exception. This code handles errors that may come up during the program’s execution. If an error occurs, the exceptions catch it and execute the code in the “catch” block.

Get More Programming and Web Scraping Help From Rayobyte

You can easily resolve syntax errors in Java compared to other types of errors. Fixing and preventing syntax errors in Java programs is as simple as reviewing notifications in your IDE and noting the specific syntax errors in Java it highlights when you attempt to run the program.

If you want to learn more about fixing syntax errors in Java, web scraping, and how to use proxies to hide your IP so you can scrape without getting blocked, visit our blog for all sorts of proxy-related information or get in touch today.

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